32S-3/3A Transmitter

THE COLLINS 32S-3/3A TRANSMITTER

The Collins highly flexible 32S-3 Transmitter covers all the
amateur bands between 3.4 mc and 30 mc with a power input of
175 watts PEP on SSB or 160 watts on CW. It has a nominal
output of 100 watts.

All of the dials and controls are clearly marked and human-
engineered for operational efficiency. In the 32S-3, Collins
design engineers have produced a transmitter with a minimum
number of controls to offer you precise tuning.

The 32S-3 can also be adapted for RTTY operation because of
the high degree of stability of the unit. Grid block keying
in CW with adjustment of characteristic from "soft" to
"hard" for individual operator is provided.

A combination of features found in no other transmitter on
the amateur market gives you that top performance which you
expect - and get - only in Collins transmitters. These
features include:

DUAL CONVERSION with the high frequency oscillator crystal-
controlled, and low frequency oscillator variable, resulting
in a frequency stability for Collins transmitters which has
become the standard of the amateur field.

COLLINS AUTOMATIC LOAD CONTROL which aids in preventing
over modulation and provides up to 10 db compression for
higher average talk power.

CW SPOTTING CONTROL which allows the 32S-3 frequency to be
quickly and easily adjusted to zero beat with a signal tuned
on the receiver. The carrier is momentarily keyed in the low
level stages without disabling the receiver. A level control
facilitates zero adjustment.

The 32S-3 comes equipped with the crystal sockets, crystals
and bandswitch positions for eleven 200 kc bands. Crystal
sockets and bandswitch position also are provided for two
additional 200 kc bands between 28 mc and 30 mc. A
fourteenth position, corresponding to the WWV position on
the receiver, can be used for an additional 200 kc band in
the 9.5-15.0 mc range, if desired. Regulation of oscillator
voltages insures high frequency stability.

The 32S-3 can be operated on any frequency in the range of
3.4-30 mc, except 5.0-6.5 mc, by installing an appropriate
crystal. Plug-in crystals are available to convert any of
the above amateur band channels to out-of-band channels.
Easy access is provided to crystal oscillator patch cables
from top of cabinet. Front panel selection of receiver VFO
or transmitter VFO provides optional transceiver operation
at the flick of a switch.

While the 32S-3 provides ample RF power for excellent
communication, it can be used without modification to excite
the Collins 30L-1 or 30S-1 Linear Amplifier.

Under the hood...

32S-3/3A CIRCUITRY

Type 32S-3A is a 175-watt input transmitter covering 3.4 to
30 MHz. The transmitter uses filter type sideband generation
and heterodyne exciter principles. A crystal-controlled bfo,
crystal-controlled high-frequency oscillator, and highly
stable vfo form a double conversion circuit. The low-
frequency if is 455 kHz, and the high-frequency if is 3.055
MHz with a 200-kHz wide passband, from 2.955 to 3.155 MHz.
The 32S-3A may be connected in transceiver service with 75S-
( ) receivers.

AF CIRCUITS

Microphone or phone patch audio is coupled in the grid of
first audio amplifier V1A, amplified, and coupled to the
grid of second audio amplifier V1B. Output from V1B is
coupled to the grid of cathode follower V2A across MIC GAIN
control. Output from the cathode follower is fed to the
balanced modulator. In TUNE, LOCK KEY, and CW positions of
the EMISSION switch, output from the tone oscillator, V11B,
is fed to the grid of the second audio amplifier. Tone-
oscillator signal is taken from the plate of V1B to the grid
of the VOX amplifier and the CW sidetone jack, J19.

BALANCED MODULATOR AND ASSOCIATED CIRCUITS

Audio output from the cathode of V2A is fed to the junction
of CR3 and CR4. In USB and LSB positions of the EMISSION
switch, the bfo voltage is fed to the junction of C187A and
C187B. (In TUNE, LOCK KEY, and CW positions of the EMISSION
switch, the bfo voltage by-passes the balanced modulator, if
amplifier, and mechanical filter and is fed directly to one
of the first mixer cathodes.) Output from the balanced
modulator consists of both upper and lower sidebands and is
coupled through if transformer T2 to the grid of if
amplifier V3. Output from if amplifier V3 is fed to
mechanical filter FL1. The passband of FL1 is centered at
455 kHz.

This passes either upper or lower sideband depending upon
the sideband polarity selected when the EMISSION switch
connects bfo crystal Y12 or Y13.

BALANCED MIXERS

The 455-kHz single-sideband signal is fed to the first
balanced mixer grids in push-pull; the plates are connected
in push-pull; and the vfo signal is fed to the grids in
parallel. The mixer suppresses the vfo signal and translates
the 455-kHz single-sideband signal to a frequency between
2.955 and 2.155 MHz. This is the bandpass if. The coupling
network between the plates of the first mixer and the grid
of the second balanced mixer is broadbanded to provide a
uniform response to the bandpass if. The bandpass if signal
is fed to one of the grids of the second balanced mixer, and
the high-frequency injection signal from the crystal
oscillator V12 is fed to the signal input cathode and to the
other grid. This arrangement suppresses the high-frequency
injection signal within the mixer and translates the
bandpass if signal to the desired operating band.

RF CIRCUITS

The slug-tuned circuits coupling V5 to V6, V6 to V7, and V7
to the power amplifier are ganged to the EXCITER TUNING
control. The signal is amplified by rf amplifier V6 and
driver V7 to drive power amplifier V8 and V9. Output from
the power amplifier is coupled by a pi-network to the
antenna through contacts of transmit-receive relay K2.
Negative rf feedback from the pa plate circuit to the driver
cathode circuit permits a high degree of linearity at the
high power level of the pa tubes. Both the driver and pa
stages are neutralized to ensure their stability.

CONTROL CIRCUITS

ALC Circuit

Detected audio-frequency voltage from the power amplifier
grid circuit is rectified by CR5 and CR6, and the negative
dc output is fed to the ALC bus. A fast attack, slow release,
dual time constant is used to prevent over-driving on
initial syllables and to hold gain constant between words.
The fast time constant ALC is applied to V6, and the slow
time constant ALC is applied to V3. If the companion 30S-1
or 30L-1 Power Amplifier is used with the 32S-3A ALC, output
from the 30S-1 or 30L-1 is fed back to the ALC bus.

VOX and Anti-VOX Circuits

Output from second audio amplifier V1B is fed to the grid of
VOX amplifier V14A through VOX GAIN control R74. This audio
input is amplified by V14A and rectified by VOX rectifier
V10B. When the positive output of V10B is high enough to
overcome the negative bias on V11A grid, the VOX relay is
actuated to turn the transmitter on. Receiver output is fed
from J13 through ANTI VOX GAIN control R85 to the grid of
anti-VOX amplifier V14V. Output from V14B is rectified by
anti-VOX rectifier V10A to provide the negative bias
necessary to keep the transmitter disabled during receive
periods. The anti-VOX circuit provides a threshold voltage to
prevent loudspeaker output (picked up by the microphone)
from tripping the VOX circuit into transmit. ANTI VOX GAIN
control R85 adjusts the value of the anti-VOX threshold so
that loudspeaker output will not produce enough positive dc
output from the VoX rectifier to exceed the negative dc
output from the anti-VOX rectifier and cause V11A to actuate
VOX relay K1. Speech energy into the microphone will cause
the positive VOX voltage to overcome the negative anti-VOX
voltage and produce the desired action of K1. Contacts of
relay K1 control relay K2, key line, PA and driver screens,
receiver muting circuits, and oscillator plate voltages.

Manual Gain Control

The MIC GAIN control is a dual potentiometer. Section R8A
controls microphone gain during SSB operation. Section R8B
is a cathode potentiometer which controls the gain of rf
amplifier V6 during CW, TUNE, or LOCK-KEY operation. This
control will be set more clockwise in these modes than it
will be in the USB or LSB modes.

Oscillators

Tone Oscillator

The tone oscillator is used for VOX circuit actuation and
sidetone generation during CW operation. It consists of an
RC phase-shift oscillator operating at approximately 750 Hz.
Its output is amplified by the second audio amplifier which
then supplies the sidetone output and also activates the VOX
circuitry to provide CW break-in. In TUNE and LOCK KEY, the
oscillator is used in conjunction with the second audio
amplifier to give sidetone output. The oscillator is turned
on when EMISSION switch section S8C is in TUNE, LOCK KEY, or
CW position.

Beat-Frequency Oscillator

The bfo is crystal controlled at either 453.650 kHz or
456.350 kHz depending upon whether Y12 or Y13 is selected by
EMISSION switch section S8F. These crystal frequencies are
on either side of the passband of mechanical filter FL1, so
the carrier frequency is placed approximately 20 dB down on
the skirts of the filter response. This carrier suppression
is in addition to the 30-dB minimum suppression provided by
the balanced modulator.

Variable Frequency Oscillator

The vfo uses fixed capacitors, a permeability tuned variable
inductor, and fixed inductors to provide the tuning range of
2.5 to 2.7 MHz. The frequency-determining network is
composed of capacitors C301, C302, C303, and C305, and
inductors L301, L302, and L303. Capacitor C303 is paralleled
by trimmer capacitor C308 and diode CR301 connected in
series. A dc bias voltage is applied to the diode through rf
isolation resistor R303. When LSB emission is selected,
negative bias is applied to CR301 which switches C308 out of
the circuit. Selection USB emission applies positive bias to
CR301, causing it to conduct which switches C308 into the
circuit. Proper adjustment of C308 shifts the vfo output
frequency by an amount equal to the frequency separation of
the two bfo crystals. This allows selection of either
sideband without changing the suppressed carrier frequency
of the exciter rf output.

High-Frequency Crystal Oscillator

High-frequency crystal oscillator V12 is crystal controlled
by one of 14 crystals (11 supplied and 3 optional) selected
by BAND switch S14, or by one of 14 crystals (none supplied)
selected by BAND switch S11. Output from the high-frequency
crystal oscillator is fed to the second mixer. This
frequency is always 3.155 MHz higher than the lower edge of
the desired transmit band. This high-frequency injection
signal is the crystal fundamental frequency for all desired
output signals below 12 MHz, but for operating frequencies
higher than 12 MHz, the crystal frequency is doubled in the
plate circuit of the oscillator.

KEYING CIRCUITS

Grid-block type keying is used for CW operation in the 32S-
3A. With the key up, a negative voltage is applied to the
grids of a second audio amplifier and the second mixer. This
prevents the amplified tone oscillator output from actuating
the VOX circuitry and also cuts off the second mixer. The
keying time constant of the second audio amplifier is fast
attack and slow release with R127 and C115 determining the
fast attack and R125 and C115 determining the slow release.
The keying time constant of the second mixer is slow attack
and slow release with the slow attack determined by R123,
R124, and C81. R123, R124, C81, and C115 determine slow-
release time. When keying takes place, second audio
amplifier and the VOX circuitry are actuated before the
second mixer. The release times of the second audio
amplifier and second mixer are approximately the same. The
VOX TIME CONSTANT control adjusts release time of the VOX
circuitry to permit fast ON-OFF keying.

Variable resistor R123 provides a choice between the
extremes of "hard" and "soft" keying. This control and its
effect are described fully in paragraph 2.1.3. Capacitor
C115 determines, to a large extent, the release time of the
"break." An additional effect is that the larger this
capacitor, the greater is the "lag" introduced which is
characterized by the bell-like type of keying well known to
CW operators. The values of C115 and R123 have been chosen
to produce generally acceptable keying. While it is not
suggested the value of C115 should be changed, if the
operator desires an increased amount of the bell-like
characteristic, a slightly larger value will produce the
effect. The additional "lag". however, will reduce the
maximum speed at which the 32S-3A may be satisfactorily
keyed.

An additional amplifier following an exciter can change
keying characteristics. A well-designed and adjusted linear
amplifier, such as the 30L-1 or 30S-1, has a negligible
effect on keying. A class C amplifier and its associated
power supply, however, will generally have considerable
effect, because cutoff bias must be overcome before the
signal is amplified, thus causing a sharp wavefront. Proper
adjustment of the keying waveshape can only be made with the
exciter driving the class C amplifier.